As miniaturization of integrated circuits continues, photolithography processes that use deep ultraviolet (DUV) radiation are often inadequate, typically at the wavelength of 193 nm. In order to support the further miniaturization, radiation with a wavelength that is an order of magnitude shorter than that of DUV radiation is required.
Developers of photolithography processes have turned to extreme ultraviolet (EUV) radiation. For example, EUV photolithography may utilize radiation at the wavelength 13.5 nm. A typical 13.5 nm radiation source includes a plasma (e.g., a laser-produced plasma) in which the radiation is produced.
A photolithography system based on EUV radiation differs in several aspects from a system based on DUV radiation. A DUV system typically involves transmitting optics (e.g., lenses). A DUV photomask typically involves a transparent substrate upon which a pattern of reflecting or absorbing material is deposited. Since EUV radiation is absorbed by materials, all EUV lithography optics are based on reflection, rather than transmission and refraction. Typical EUV optics is based on multilayer reflectors that reflect radiation as determined by Bragg's Law. The multilayer reflectors typically include a large number (e.g., 40 to 50, or more or fewer) of alternating layers of molybdenum (Mo) and silicon (Si). The reflectors are used in focusing and beam-directing optics, as well as in the photomask itself. A typical EUV photomask includes reflector upon which an absorbing material is deposited.
Process variations in lithographic printing of integrated circuits are characterized by variations in the sizes of structures (e.g., lines and spaces) printed on a substrate such as a silicon wafer. Variations of printed lines and spaces are measured as an average critical dimension (CD) size at different areas of the wafers. Variations in the width of lines and spaces imprinted on a wafer may occur, for example, due to photomask imperfections. Other sources of CD variations on the wafer may include, for example, unevenness on a chuck that holds a wafer or variations in the photolithography process.